This invention relates to a device for correcting the convergence of an electron beam in a color television receiver and in particular to a digital convergence correcting device used in the case where a high precision convergence correction is required.
A prior art digital convergence correcting device is disclosed in JP-B-56-40355. According to the invention disclosed in this publication, convergence correcting waveforms (correcting data) are stored in a memory and the correcting data corresponding to the position of the scanning line are read out from the memory to correct the convergence. This prior art example is shown in FIG. 11.
In FIG. 11 convergence correcting waveforms corresponding to the position of the scanning line on the image screen are stored as correcting data in a memory 1. These correcting data stored in the memory 1 are read out in synchronism with the scanning of the image screen and supplied to a D/A converter 2. The correcting data are converted into analog signal in the D/A converter 2. Thus, a continuous analog correcting waveform signal is outputted through a low pass filter (LPF) 3. Further, the output signal of this LPF is inputted to a convergence yoke 5 through a voltage-current converting amplifier 4.
The prior art digital convergence correcting device is used only in there industrial color television receivers, etc., for which a high precision correction was required. For this reason the prior art convergence correcting device was is used in a color television receiver whose input signal is standardized. Consequently, heretofore no attention has been paid to the convergence correction for television signals produced at special reproduction, etc. in VTRs, video disc devices for home use, etc., different from those that are standardized.
For example, for high speed reverse direction picture searches in VTR's, the number of scanning lines on the image screen constituting one field increases. The correcting data stored in the memory 1 are formed on the basis of a standard signal for which the number of scanning lines is predetermined. For this reason, in the prior art convergence correcting device it is not possible to effect the convergence correction corresponding to the increase in the number of scanning lines for high speed reverse direction picture search. As the result, misconvergence takes place in the lower part of the image and image quality is significantly deteriorated.
Hereinbelow, the reason why the number of scanning lines on the image screen constituting one field increases for high speed reverse direction picture searches in VTRs, for home use will be explained, referring to FIGS. 12(a), 12(b), 13(a) and 13(b).
FIG. 12(a) is a scheme for explaining video tracks 11 for the image signal recorded on a video tape 10 for a VTR. In the figure, for normal reproduction a video head 12 scans the tape as indicated by an arrow 13 to reproduce the image signal recorded on the video track 11. Information corresponding to one field is recorded in the video track 11. Denoting the displacement velocity of the video tape 10 at this time by v.sub.t and the peripheral velocity of the cylinder (not shown in the figure) around which the tape is wound, by v.sub.cl the velocity of the video head 12 with respect to the tape can be represented by the vector sum of v.sub.t and v.sub.cl.
Next, the state in which the video tape is scanned in the reverse direction is indicated in FIG. 13(a). The video head 12 scans the tape, as indicated by arrow 14, and reproduces the image signal on the video track 11 along an oblique direction. The velocity of the video tape 10 at this time is represented by -v.sub.t.
Accordingly, in the state indicated in FIG. 13(a), the velocity of the video tape 10 --v.sub.t, the peripheral velocity of the cylinder v.sub.c2 and the velocity of the video head 12 with respect to the tape v.sub.r have a vectorial relationship as indicated in FIG. 13(b).
In this case, the scanning length in FIG. 13(a) is longer than in FIG. 13(b) and the number of scanning lines in one field increases.
In the above, for the description purpose of simplifying explanation, it was assumed that the image is reproduced with the same speed in the reverse direction. The above explanation is valid also for a high speed reverse direction picture search. Consequently, with increasing of the speed of the picture search in the reverse direction, the number of scanning lines in one field increases. In a usual VTR the number of scanning lines can increase by about 20 lines. That is, the number of scanning lines in one field can be greater than 260.
It is a matter of course that the number of scanning lines in one field decreases for a forward direction picture search, contrarily with respect to that in the reverse direction.
For this reason, in the case where correcting data for the digital convergence correction corresponding to the number of scanning lines for the standard image signal are stored in the memory, the state in which correcting data are insufficient or excessive can take place.
As described above, the prior art industrial digital convergence correcting device has a problem as follows: the convergence correction is impossible for the image signals whose number of scanning lines is increased or decreased for special reproduction in VTRs, etc., which gives rise to the problem that misconvergence takes place in the part of the image screen corresponding to the increase or decrease in the number of scanning lines and the image quantity is deteriorated.